Which one of the following statements is correct regarding when firms exercise their call options on bonds?

A. if the interest rate is high, the firm can call a bond so that it can issue new bonds at a higher interest rate

B. if the interest rate is low, the firm can retire a bond and issue new bonds at a lower rate interest rate

C. if the interest rate is low, the firm would call a bond and pay investors the market price of the bond

D. if the interest rate is high, the firm would call a bond because the price of the bond has fallen